Treatment as Prevention: Do the Individual Prevention Benefits Translate to the Population Level?

Antiretroviral treatment can dramatically improve the long-term health of someone living with HIV. It can also reduce their risk of onward HIV transmission. Therefore, efforts to increase the number of HIV-positive people who are on treatment may -- as a secondary benefit to improving health -- help curb the number of new HIV infections in Canada and around the world. The idea that treatment may be able to prevent HIV transmission at the population level and have an impact on the HIV epidemic is commonly referred to as "treatment as prevention." This has led to the concept of a "test and treat" or "seek, test and treat" strategy: a public health approach with the goal of maximizing the prevention benefits of treatment by increasing treatment coverage and decreasing community viral load.

Antiretroviral treatment (ART) can reduce the viral load in the blood, semen, vaginal fluid and rectal fluid to very low levels (undetectable by current viral load tests) and this can reduce an individual's risk of HIV transmission. This was demonstrated in a randomized controlled study known as HPTN 052 where treatment reduced the risk of HIV transmission by 96% among heterosexual serodiscordant couples.1 However, it is important to point out that couples in this study reported mostly having vaginal sex and were provided with regular adherence counselling, viral load tests, testing and treatment for sexually transmitted infections (STIs), and prevention counselling and free condoms. All of these may have helped maximize the prevention benefit of treatment.

While it's unclear if the same reduction in risk applies to other populations -- such as couples who do not receive these ongoing services, people in casual relationships, gay men and other men who have sex with men (MSM), and people who use injection drugs -- it is generally thought that ART reduces HIV risk to some extent.2

Prevention Benefits at a Population Level?

Since treatment has prevention benefits at the individual level, it makes sense to think it may also have prevention benefits at the population level. In other words, increasing the number of HIV-positive people on treatment could lower the total amount of virus circulating in a community (also known as community viral load) and lead to a reduction in the number of new HIV infections.

So what evidence do we have? As HIV testing and treatment have become more accessible over the past decade, the number of HIV-positive people on treatment has been steadily increasing in most parts of the world. Whether the prevention benefits of treatment have already translated to the population level (prior to the implementation of any specific "seek, test and treat" strategies) can be explored by looking at whether increases in treatment coverage -- and subsequent decreases in community viral load -- have coincided with a reduction in new HIV diagnoses or estimated number of new HIV infections (also known as HIV incidence).

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However, the evidence available so far is observational and has its limitations. Even if it is found that the number of HIV diagnoses (or HIV incidence) decreased in a population as more people accessed HIV treatment (a positive finding), it is difficult to know exactly why this happened. For example, a decrease in HIV diagnoses may have been the direct result of increased treatment coverage or it may have been the result of other HIV prevention interventions that were implemented at the same time.

Conversely, if it is found that HIV diagnoses stayed the same or increased as treatment coverage increased (a null finding), it is also difficult to know why this happened. For example, this may mean that treatment did not have prevention benefits or, on the other hand, it is possible that treatment did prevent transmissions but improvements in HIV testing services led to the identification of more cases of HIV and made it appear that treatment did not have an effect.

Furthermore, community viral load may not be a good indicator of overall infectiousness in a population. This is because community viral load is generally calculated using the viral load measurements of people living with HIV who are diagnosed and in care. Therefore, this measure does not include those who are undiagnosed or not in care and, as a result, may not reflect the viral load among all HIV-positive people in a community.

Because of these limitations, the evidence available so far should be viewed cautiously.

Below is a summary of the evidence of whether treatment has had a prevention impact in different populations.

Heterosexual Populations in Low-to-Middle Income Countries

Globally, several countries have experienced decreases in estimated HIV incidence as treatment coverage has increased, suggesting treatment may be having a prevention impact.3 According to UNAIDS, HIV incidence decreased by more than 25% in 39 countries -- the majority of which were low-to-middle income, in sub-Saharan Africa, and have generalized heterosexual epidemics -- between 2001 and 2011. During the same time period, treatment coverage increased dramatically in many of these countries. For example, treatment coverage in sub-Saharan Africa increased by more than 100-fold. Although other HIV prevention efforts were also dramatically scaled up during this time, some experts think increased treatment coverage may have contributed to the decrease in new HIV infections.4

Furthermore, a recent study of a group of over 16,000 HIV-negative individuals in a rural area in South Africa found that for every 1% increase in ART coverage among people living with HIV, the risk of HIV infection decreased by 1.4%.Furthermore, an HIV-negative person living in a community with treatment coverage of 30-40% was on average 38% less likely to become infected than if they were living in a community with coverage less than 10%.5

People Who Use Injection Drugs in British Columbia

"Treatment as prevention" may be occurring among people who use injection drugs in British Columbia. Between 1996 and 2009, the number of people with HIV on ART in B.C. increased from 837 to 5413 (a 550% increase) and -- in the same time period -- the number of new HIV diagnoses fell from 702 to 338 per year (a 50% decrease).6 However, HIV diagnoses only decreased among people who use injection drugs and not among non-injection drug using populations. In a separate Vancouver-based study, decreased community viral load was associated with reduced estimated HIV incidence among a group of HIV-negative drug users.7

Even though reductions were observed among injection drug users in these studies, some experts have noted that other services for this population were improved during this time and may have contributed to this decrease.8 For example, the Vancouver-based supervised injection site known as Insite opened its doors in 2003.

Gay Men and Other MSM in High-Income Countries

"Treatment as prevention" does not appear to be happening among many populations of gay men and other MSM. In most high-income countries, estimated HIV incidence among MSM is remaining stable or continuing to increase despite increases in treatment coverage and decreases in community viral load.9,10

For example, in England and Wales, treatment uptake among diagnosed MSM in care rose from 69% in 2001 to 80% in 2010 and an estimated 53% of all MSM living with HIV in 2010 had an undetectable viral load.12 However, the annual number of new HIV diagnoses remained unchanged during this time period. Treatment coverage levels among MSM in France and Australia are similar to those in the UK but the number of new HIV diagnoses are continuing to increase in these countries.13,14 While the number of new HIV infections does not appear to be decreasing, experts think HIV incidence among MSM in many high-income countries would be higher if not for increased treatment coverage.15-18

An exception is the experience of San Francisco, where increased treatment coverage and decreased community viral load has been associated with a reduction in new HIV diagnoses.19 Between 2004 and 2008 the proportion of diagnosed HIV-positive MSM in care with an undetectable viral load increased from 45% to 78% (resulting in a decline in community viral load) and the number of new HIV diagnoses decreased from 798 to 434 (a 54% decrease). A more recent study -- extending this time period to 2011 -- supports the idea that "treatment as prevention" is occurring among gay men in this city.20

Other Studies

Increased treatment coverage has not reduced HIV incidence in Washington, D.C. Between 2004 and 2008, the proportion of diagnosed HIV-positive individuals in care with an undetectable viral load increased from 15% to 58%, the community viral load decreased, but the annual number of new HIV diagnoses did not change.21

In Southern Alberta, treatment coverage among people living with HIV in care increased from 62% to 81% between 2001 and 2010, while the proportion of people with a viral load of less than 200 copies/ml increased from 49% to 72%. Despite the large increase in treatment coverage and viral suppression, community viral load surprisingly remained stable and the number of new HIV diagnoses increased.22

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